The present invention relates to a method and nozzle for producing fine powder, preferably having a spherical physical appearance, by atomizing molten material with gases such as known for EP-A-0444-767.
To produce metal powders, gas atomization techniques are known throughout the industry. Different nozzle constructions are utilized, all of which have in common that a pressurized atomization gas escapes from one or more gas nozzles and, as a turbulent stream, approaches at an angle molten material flowing out of a molten material nozzle and atomizes such molten material. An overview of various nozzle constructions is provided, for example, by A. J. Yule and J. J. Dunkley xe2x80x9cAtomization of Meltsxe2x80x9d, Oxford, 1994, pages 165 to 189. On its way to the molten material, the gas loses a large portion of its energy. With atomization gas pressures up to about 35 bar, relatively coarse metal powder having average granular diameters d50 in the atomization state of about 50 xcexcm and greater result. The thus produced powders generally have a broad granular size distribution because the atomization pulse is subjected to great deviations due to the turbulence. J. Ting, et al., xe2x80x9cA novel high pressure gas atomizing nozzle for liquid metal atomizationxe2x80x9d, Adv. Powder Metallurgy and Particulate Materials, 1996, pages 97 to 108, discloses special high pressure nozzles having operating pressures of up to 100 bar, which at a very high gas consumption can produce average granular sizes of about 20 xcexcm. All known methods having turbulent gas flow are unsuitable for the direct production of fine powders having average granular diameters d50 of about 10 xcexcm.
DE 33 11 343 A1 discloses a method of producing fine metal powders as well as a device for carrying out the method, and proposes the use of laminar gas streams in a concentric Laval nozzle having preheated atomizing gas. The molten material nozzle is positioned in such a way that it is disposed in the converging portion of the Laval nozzle, i.e., that the molten material nozzle extends into the Laval nozzle. The flow in the upper portion of the Laval nozzle is laminar. In contrast to methods having turbulent gas flows, finer powder with a narrower granular size distribution, accompanied by relatively low specific gas consumption, result, as illustrated, for example, in FIG. 2 of the publication of G. Schulz, xe2x80x9cLaminar sonic and supersonic gas flow atomizationxe2x80x9d PM2TEC ""96, World Congress On Powder Metallurgy And Particulate Materials, U.S.A., 1996, pages 1 to 12. The specific gas consumption for the production of a steel powder having an average granular diameter of 10 xcexcm is approximately 7 to 8 Nm3Ar/kg corresponding to about 12.5 kg to 14.2 kgAr/kg steel.
DE 35 33 964 C1 discloses a method and an apparatus for producing very fine powders in spherical form, according to which the atomizing gas is introduced via a radially symmetrical, heatable gas hopper into the Laval nozzle, whereby the metal exiting the molten material nozzle, which is placed within this gas hopper, is overheated or heated by heat transfer via radiation, which originates from the heated gas hopper.
DE 37 37 130 A1 similarly discloses a method and an apparatus for producing very fine powders, according to which the underpressure resulting from the gas flowing in the Laval nozzle is utilized to draw in molten material from a separate molten material device. Here also a radially symmetrical nozzle system having a molten material nozzle placed within the Laval nozzle is involved.
From the publication of G. Schulz, xe2x80x9cLaminar sonic and supersonic gas flow atomizationxe2x80x94The NANOVALxe2x80x94Processxe2x80x9d, Adv. Powder Metall. and Particulate Matter. (1996), 1, pages 43-54, it is furthermore known that for the production of fine metal powder it is necessary to keep the mass flow exiting the radially symmetrical nozzle small if fine powder is to be produced. Indicated here are 12 to 30 kg/h and nozzles with molten material nozzle diameters of 1 mm or less.
Common to all of the previously known methods is that these have serious technical and economical drawbacks. For example, the heretofore utilized concentric or radially symmetrical nozzle systems having molten material nozzle diameters of 1 mm or less, are, due to the type of construction, particularly susceptible to mechanical clogging due to foreign particles or gas bubbles that are carried along. In addition, due to a given unfavorable ratio of outer molten material nozzle surfaces to the molten material volume, great heat losses occur that can effect an undesired congealing of the molten material nozzles and then, as is also the case with the mechanical clogging, result in a termination of the atomization and longer down times. Furthermore, the production capacity that up to now could be achieved is low, and the specific gas consumption is high. During the production of fine powders, the production capacity and the specific gas consumption are very decisive in determining the manufacturing costs. There is therefore a need for an atomizing method that is characterized by low gas consumption and high production capacity.
Taking into account this state of the art, the object of the invention is to improve a method of the aforementioned general type, while avoiding the described drawbacks, in such a way that an economical production of fine, gas-atomized powder is possible. Furthermore, down times due to clogging from impure molten material, and from congealing due to heat losses, are to be avoided. In particular, it should be possible to finely and uniformly atomize metallic, metallic alloy, salt, salt mixture, or also polymeric molten material on a large scale, in an economical manner, and in particular, however, with a low gas consumption and a high molten material throughput. Furthermore, the molten material nozzle should be as stable as possible relative to mechanical clogging from impure molten material as well as relative to congealing.
The object is inventively realized in that the molten material flows out of a molten material nozzle having an essentially rectangular cross-sectional area in the form of a film, and subsequently, together with an atomizing gas, issues through an initially converging and then diverging gas nozzle that is in the form of a linear Laval nozzle, has an essentially rectangular cross-sectional area, and through which flow is laminar, whereby the laminar accelerated gas flow stabilizes and simultaneously stretches the film of molten material in the converging portion of the Laval nozzle until the film of molten material, after passing the narrowest cross-sectional area, is uniformly atomized over its entire length.
Surprisingly, it is possible to stabilize the film of molten material, which is primarily issuing from the essentially rectangular molten material nozzle, and that would be unstable due to its large surface area by virtue of free discharge, by the introduction into the accelerated gas stream in the converging portion of the similarly essentially rectangular Laval nozzle. In so doing, an extremely favorable relationship of powder molten material nozzle surface to the molten material volume is achieved, so that clogging due to congealing is precluded. Furthermore, individual foreign particles in impure molten material can in the most unfavorable situation affect only a small portion of the cross-sectional area of the molten material nozzle, so that the atomizing process is also not terminated under such conditions. Below the narrowest cross-sectional area of the Laval nozzle, the film of molten material is uniformly atomized with high specific pulse to a fine powder that preferably has a spherical physical appearance.
Pursuant to a further advantageous proposal of the invention, the ratio of the pressure above the Laval nozzle and below the Laval nozzle corresponds at least to the critical pressure ratio of the atomizing gas that is utilized, so that the gas reaches the speed of sound in the narrowest cross-sectional area of the Laval nozzle. The pressure ratio is advantageously  greater than 2, preferably  greater than 10.
Pursuant to a further advantageous proposal of the invention, the atomizing gas is preheated. Pursuant to a further advantageous proposal the molten material issuing from the molten material nozzle is heated via radiation. The preheating of the atomizing gas, and the heating-up of the molten material via radiation, are, however, not necessary prerequisites for being able to carry out the method. A preheating of the atomizing gas, and a heating-up of a molten material issuing from the molten material nozzle via radiation, are preferably dispensed with, as a result of which on the one hand the capital outlay for equipment is considerably reduced, and on the other hand energy is saved.
Pursuant to a further advantageous proposal of the invention, impure molten material is also atomized by the molten material nozzle. Metals, metal alloys, salts, salt mixtures, or synthetic materials that can be melted, such as polymers, are advantageously utilized as molten material that is to be atomized.
Pursuant to a further advantageous proposal of the invention, the molten material that is to be atomized does not react with the atomizing gas, in other words, is inert relative to the gas. If the material that is to be atomized does not react with the atomizing gas, in other words is inert relative to the gas, spherical particles form from the molten material droplets under the influence of surface tension. Pursuant to a further proposal of the invention, the molten material that is to be atomized reacts entirely or partially with the atomizing gas. If the material that is to be atomized, in other words the molten material, reacts entirely or partially with the atomizing gas, reaction products are thereby formed that can prevent the molten material droplets from forming into spheres, so that irregularly formed powder particles are formed. If a substrate is advantageously introduced into the particle stream at a distance at which the particles are at least still partially liquid, the direct production of a semi-finished product is possible, a so-called atomizing compaction.
Pursuant to the method, the ratio of the cross-sectional surfaces of the molten material nozzle discharge to the narrowest cross-sectional area of the Laval nozzle with linear systems is always greater than with radially symmetrical nozzles. Since the throughput quantities of gas and metal and the like under otherwise identical conditions are proportional to the corresponding nozzle cross-sectional areas, pursuant to the method there results linear systems having lower specific gas consumptions. The savings increases with the length of the nozzle system. Due to the proportionality of molten material nozzle cross-sectional area and molten material throughput, by adaptation of the nozzle length every desired production capacity can be set in a simple manner. The characteristic properties of metal powder, such as granular size, width of the granular size distribution, and granular shape, thereby remain unchanged, in contrast to which pursuant to the method the specific gas consumption drops.
As a device for carrying out the method of the invention a nozzle for atomizing molten material is inventively proposed that has a molten material nozzle and a gas nozzle disposed below the discharge thereof in the direction of flow, with the inventive nozzle being characterized in that the molten material nozzle has an essentially rectangular discharge cross-sectional area, in that the gas nozzle also has an essentially rectangular cross-sectional area in the form of a linear Laval nozzle, and in that the gas nozzle generates an initially converging, laminar accelerated gas flow that stabilizes and simultaneously stretches the film of molten material until after passing the narrowest cross-sectional area in the diverging portion of the gas nozzle the film of molten material is uniformly atomized over its entire length. As a consequence of the inventive nozzle with an essentially rectangular cross-sectional area, in other words, a rectangular or substantially rectangular cross-sectional area, the cross-sectional area can, by altering the length of the rectangle, be adapted in such a way that every desired molten material throughput can be achieved and in this way a high production capacity results.
Pursuant to one advantageous proposal of the invention, the discharge cross-sectional area of the molten material and/or the Laval nozzles are modified in such a way that the two short sides of the rectangle of the nozzle cross-sectional area are replaced by semi circular arcs having a diameter corresponding to the length of the short sides, so that a substantially rectangular cross-sectional area is provided.
Pursuant to a further particularly advantageous proposal of the invention, the ratio of the long side of the rectangle and the short side of the rectangle of the discharge cross-sectional area of the molten material and/or of the Laval nozzles is at least  greater than 1 preferably  greater than 2, and especially preferably  greater than 10. Pursuant to a further advantageous proposal of the invention, the length of the linear Laval nozzle in the narrowest cross-sectional area is greater than the length of the molten material nozzle. Advantageously, the ratio of the width of the Laval nozzle to the width of the molten material nozzle  greater than 1 and  less than 100, preferably  less than 10.
Pursuant to a further particularly advantageous proposal of the invention, the molten material throughput is adapted to the desired production capacity by lengthening the long side of the molten material nozzle and corresponding lengthening of the long side of the Laval nozzle by the same amount, without thereby altering the granular size of the powder that is to be produced or increasing the specific gas consumption.